Grass-Legume Mixed Cover Crops for Weed Management
Weed management is a vital component of successful crop production. Crop productivity in modern times relies heavily upon the efficacy of pesticides to manage insects, diseases, and weeds. In U.S. agriculture, herbicides generally comprise the major constituent of pesticide inputs in major field crops, including cotton (Gossypium hirsutum L.), corn (Zea mays L.), and soybean [Glycine max (L.) Merr.] (see Table 5.1). Use of insecticides and fungicides in field crops varies widely from 1 and 0 percent, respectively, in soybean to 93 and 85 percent, respectively, in fall potatoes (Solanum tuberosum L.). Insecticides are used more heavily on vegetable crops (76 percent) compared to herbicides (59 percent) (USDA, 2003). Fungicides are also used more intensively on vegetable crops than on field crops. Sustainable agriculture requires good stewardship of pesticide use and development of ways to reduce pesticide input. Weed management with reduced herbicide inputs is possible by integrating complementary technologies. These include reduced tillage and use of allelopathic cover crops and mulches, among other strategies. Many allelopathic cover crops, with high biomass production and good manageability, are grasses such as annual ryegrass (Lolium multiflorum Lam.), barley (Hordeum vulgare L.), oat (Avena sativa L.), rye (Secale cereale L.), and wheat (Triticum aestivum L.) (Nelson, Kahn, and Roberts, 1991; Fujii, 2001). Rye and wheat are more consistent in terms of cover crop biomass production (Williams, Morten-
sen, and Doran, 1998) and most reliable for early-season weed suppression (Weston, 1990). Crimson clover (Trifolium incarnatum L.) and hairy vetch (Vicia villosa Roth) are among the best legume cover crops for ground cover, weed suppression, and manageability. However, crimson clover and hairy vetch produce less biomass and are more difficult to kill with nonselective, burndown herbicides than the grassy species (Nelson, Kahn, and Roberts, 1991). Biomass production is important in choosing a cover crop because, up to a point, biomass is directly related to the degree of soil protection, weed suppression, and nutrient recycling or nutrient contribution to the following crop. Other vetch species, such as Chinese milk vetch (Astragalus sinicus L.), have been used as green manure crop in rice (Oryza sativa L.) paddies in China, Japan, and Korea (Yasue and Tsutiya, 1982; Seong and Park, 1991; Yasue, 1991; Lee et al., 1999). Chinese milk vetch matures earlier than hairy vetch and narrow leaf vetch [Vicia angustifolia (L.) Richard]. This makes Chinese milk vetch a better fit in an alternative rice-based cropping system. In Korea, cultivated areas planted to Chinese milk vetch reached about 37,000 ha, and this is expected to increase in the future (Jeong et al., 1996; Hong et al., 1997). This chapter discusses the utility of temperate small-grain cereal and legume mixtures for weed management
BENEFITS OF MIXED COVER CROPS
Grass and legume cover crops offer different, but complementary, benefits to the succeeding crop. Grass cover crops produce a higher amount of biomass (Wagger, 1989b; Nelson, Kahn, and Roberts, 1991; Yenish, Worsham, and York, 1996), conserve more residual N during fall and winter (Evanylo, 1991; Meisinger et al., 1991; Shipley, Meisinger, and Decker, 1992), decompose more slowly (Wagger, 1989b; Mohler and Teasdale, 1993;), suppress weeds better (Nelson, Kahn, and Roberts, 1991; Wallace and Bellinder, 1992; Burgos and Talbert, 1996), and are easier to kill with nonselective herbicides compared to legume cover crops. A summary of biomass production of rye, wheat, clover, and vetch planted alone or in a mixture is presented in Table 5.2. This excludes data from studies not involving comparisons of cover crop mixtures with monocultures but shows the general trend of greater biomass production from small-grain cereal than legume cover crops. Research at Fayetteville and Kibler, Arkansas, showed that rye could produce 12 mg·ha-1 shoot biomass when desiccated at booting to anthesis in late April to early May (Burgos, 1994). Hairy vetch, on the other hand, produced an average of 9 mg·ha-1 biomass. In Blacksburg, Virginia, similar experiments showed rye produced 7.7 mg·ha-1 shoot biomass on average when desiccated between the second and third week of May (Vaughan and Evanylo, 1998). In this location, hairy vetch produced almost four times less biomass than did rye. Understandably, cover crop biomass production depends upon soil, seeding rate, stand establishment, environmental conditions during the growing period, as well as timing of cover crop kill.